As an engine's compression ratio is increased, while maintaining a particular bore-to-stroke ratio, the surface to volume ratio at top dead center (TDC) increases, the temperature increases, and the pressure increases. This has three major consequences: 1) heat transfer from the combustion chamber increases, 2) combustion phasing becomes difficult, and 3) friction and mechanical losses increase. Heat transfer increases because the thermal boundary layer becomes a larger fraction of the overall volume as the aspect ratio (i.e., the ratio of the bore diameter to the length of the combustion chamber) at TDC gets smaller. Both combustion phasing and achieving complete combustion present challenges because of the small volume realized at TDC. Increased combustion chamber pressure directly translates to increased forces acting on components of the engine. These large forces may overload both the mechanical linkages within the engine (e.g., piston pin, piston rod, crank shaft) and the pressure-energized rings, thus causing increased friction, wear, and/or failure.
A primary challenge associated with linear piston engines is efficiently converting the kinetic energy of a piston to mechanical work and/or electrical energy. The space between the piston and the cylinder wall, referred to herein as a “clearance gap,” is critical in maintaining piston alignment, preventing piston-wall contact and associated friction losses, and controlling gas leakage past the piston (e.g., blow-by). The clearance gap may be affected by imbalanced forces acting on the piston, thermally induced expansion or contraction (e.g., solid deformation), changing engine conditions, or other relevant factors. Management of the clearance gap, piston temperature, cylinder temperature, or combinations thereof, may be desired in some applications.